Smart electronic switch

ABSTRACT

A circuit includes a monitor circuit. The monitor circuit includes a nonlinear functional unit configured to receive a current sense signal and to generate a power signal representing the power of the current sense signal. The circuit further includes a first filter configured to receive the power signal and to generate a first filtered signal and a second filter configured to receive an input signal that depends on the current sense signal and to generate a second filtered signal. A comparator circuit is configured to receive the first filtered signal and the second filtered signal and to compare the first filtered signal with a first threshold value and the second filtered signal with a second threshold value. The protection signal is indicative of whether the first filtered signal exceeds the first threshold value or the second filtered signal exceeds the second threshold value.

This Application claims priority to German Patent Application Number102020122571.7, filed Aug. 28, 2020, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to the field of smart semiconductorswitches.

BACKGROUND

Almost every electric installation (e.g. in an automobile, in a house,electric subsystems of larger installations) includes one or more fusesto provide over-current protection. Standard fuses include a piece ofwire that provides a low-ohmic current path in case the current passingthrough the fuse is below a nominal current. However, the piece of wireis designed to heat up and melt or vaporize when the current passingthrough the fuse exceeds the nominal current for a specific time. Oncetriggered, a fuse must be replaced by a new one.

Fuses are being increasingly replaced by circuit breakers. A circuitbreaker is an automatically operated electrical switch designed toprotect an electrical circuit from damage caused by overcurrent,overload or short-circuit. Circuit breakers may includeelectro-mechanical relays which are triggered to disconnect theprotected circuit from the supply when an over-current (i.e. a currentexceeding the nominal current) is detected. In many applications (e.g.in the on-board power supply of an automobile), circuit breakers may beimplemented using an electronic switch (e.g. a MOS transistor, an IGBTor the like) to disconnect the protected circuit from the supply in caseof an over-current. Such electronic circuit breakers may also bereferred to as electronic fuses (e-fuses or smart fuses). Besides itsfunction as a circuit breaker, an electronic fuse may also be used toregularly switch a load on and off. Usually the switching state (on/off)of electronic switches such as MOS transistors is controlled usingso-called driver circuits or simply drivers (gate drivers in case of MOStransistors).

Usually, conventional fuses—and electronic fuses—are designed for a(hypothetical) constant electric load that produces a specific thermalload on the cable. That is, the constant electric load results in aspecific cable temperature above ambient temperature. The purpose of thefuse is to ensure that the thermal load on the cable stays within adefined limit. Therefore, known electronic fuse circuits are designed toemulate the time-current characteristic of a cable that supplies theload (which defines for how long a specific current level may flowthrough the electronic fuse before the fuse triggers the disconnectionof the load). However, in many applications the load changesdynamically. In view of the fact that the thermal time constant ofcommonly used cables is in the range of a few minutes (e.g. 90 secondsin some applications) the activation of an electric load for, e.g., 30seconds, may be a highly dynamic process as compared to the thermal timeconstant of the cable.

SUMMARY

An circuit that can be employed in a smart switch is described herein.In accordance with one embodiment the circuit includes a monitor circuitthat is configured to receive a current sense signal and to provide aprotection signal. The monitor circuit includes a nonlinear functionalunit configured to receive the current sense signal and to generate apower signal representing the power of the current sense signal. Thecircuit further includes a first filter configured to receive the powersignal and to generate a first filtered signal and a second filterconfigured to receive an input signal that depends on the current sensesignal and to generate a second filtered signal. A comparator circuit isconfigured to receive the first filtered signal and the second filteredsignal and to compare the first filtered signal with a first thresholdvalue and the second filtered signal with a second threshold value. Theprotection signal is indicative of whether the first filtered signalexceeds the first threshold value or the second filtered signal exceedsthe second threshold value.

Furthermore, a method that may be used in a smart switch is described.In accordance with one embodiment the method includes providing a signalrepresenting a load current passing through a power transistor andgenerating a protection signal based on the current sense signal.Therein, generating the protection signal includes generating a powersignal representing the power of the current sense signal by applying anonlinear function to the current sense signal; filtering the powersignal to generate a first filtered signal and generating a secondfiltered signal based on the current sense signal; and comparing thefirst filtered signal with a first threshold value and the secondfiltered signal with a second threshold value. The protection signal isindicative of whether the first filtered signal exceeds the firstthreshold value or the second filtered signal exceeds the secondthreshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments described below can be better understood with referenceto the following drawings and descriptions. The components in thefigures are not necessarily to scale; instead emphasis is placed uponillustrating the principles of the invention. Moreover, in the figures,like reference numerals designate corresponding parts. In the drawings:

FIG. 1 schematically illustrates one example of an electronic fusecircuit including an electronic switch and a control circuit configuredto drive the electronic switch and an exemplary application of theelectronic fuse circuit.

FIG. 2 illustrates one example of the control circuit of FIG. 1 in moredetail.

FIG. 3 illustrates one example of a logic circuit used in the controlcircuit of FIG. 2 .

FIG. 4 shows timing diagrams illustrating the function of the controlcircuit shown in FIG. 2 .

FIG. 5 a is a diagram illustrating a family of characteristic curves(time over current) for a 0.35 mm² cable and for different maximum cabletemperatures.

FIG. 5 b is a diagram illustrating a family of characteristic curves(time over current) for a maximum cable temperature of 25 Kelvin aboveambient temperature and for different cable cross-sections.

FIG. 6 illustrates one example of the monitoring circuit used in theexample of FIG. 2 ;

the monitoring circuit includes a filter and a comparator, wherein thefilter time constant and the comparator threshold determine thetime-current-characteristic of the monitoring circuit.

FIG. 7 illustrates a first example of an e-fuse (“smart fuse”) circuitthat allows selection of wire cross section and maximum cabletemperature.

FIG. 8 is a diagram illustrating the effect of the filter time constanton the time-current characteristic in case a first order low-pass-filteris used in the monitoring circuit of FIG. 6 .

FIG. 9 is a diagram illustrating the effect of the comparator thresholdon the time-current characteristic in a case in which a first orderlow-pass-filter is used in the monitoring circuit of FIG. 6 .

FIGS. 10 and 11 illustrates two alternative embodiments of a monitoringcircuit, both of which use a plurality of filters and comparators withdifferent filter time constants and, respectively, different comparatorthresholds.

FIG. 12 illustrates one example of a complex time-current-characteristicthat can be achieved with the embodiments of FIGS. 10 and 11 .

FIG. 13 illustrates another embodiment of a monitoring circuit.

FIG. 14 illustrates another embodiment in which part of the monitoringcircuit is implemented in an external circuitry such as amicrocontroller.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings. The drawings form part of the description and,for the purpose of illustration, show examples of how the invention canbe used and implemented. It is to be understood that the features of thevarious embodiments described herein may be combined with each other,unless specifically noted otherwise. Further, although the examplesdescribed herein are directed to electronic fuse circuit, theembodiments are not limited to applications related to electronic fuses.

FIG. 1 illustrates one example of an electronic circuit that can beoperated as an electronic fuse. Therefore, the electronic circuit isfurther referred to as electronic fuse circuit F. In accordance with thepresent example, an electronic fuse circuit includes an electronicswitch 2 with a control node 21 and a load current path between a firstload node 22 and a second load node 23. The electronic circuit furtherincludes a control circuit 1 coupled to the control node 21 of theelectronic switch 2 and configured to drive the electronic switch 2. Theelectronic fuse circuit F with electronic switch 2 and control circuit 1may be monolithically integrated in one semiconductor die (chip) or maybe integrated in two semiconductor dies that are arranged in oneintegrated circuit package. Alternatively, gate driver and MOSFET may beintegrated in separate chips. The electronic fuse circuit F isconfigured to drive a load Z (the wires connecting the load areillustrated in dashed lines in FIG. 1 ), which can be connected inseries with the load current path of the electronic switch 2. Thus theseries circuit of the load current path of the electronic switch 2 andload Z can be connected between supply nodes, at which a first supplypotential and a second supply potential can be provided. The secondsupply potential is usually referred to as ground potential GND (e.g.zero volts). In the following, a voltage between the two supply nodes isreferred to as supply voltage VB. The load current i_(L) passing throughthe load Z can be switched on and off in accordance with an input signalS_(IN) supplied to the control circuit 1, for example, by a microcontroller 8. However, dependent on the application, the input signalS_(IN) can also be generated by any other circuitry instead of a microcontroller.

In an exemplary application, the electronic fuse circuit F may be usedto drive a load Z in an automobile. In this case, the power source thatsupplies the supply voltage V_(B) is an automobile battery. Generally,“to drive a load” may include switching on or off the load currentpassing through the load by switching on or off the electronic switch 2.The load may be an arbitrary load used in an automobile. Examples of theload Z include, inter alia, different types of lamps, different types ofmotors, relays, a heating system, or the like. The load Z may alsorepresent an electric subsystem (including a plurality of individualelectric loads) of the electric installation of an automobile. In theexample of FIG. 1 , the electronic switch 2 and the load Z are connectedin a high-side configuration. That is, the load Z is connected betweenthe electronic switch 2 and the ground node GND. This, however, is onlyan example. The electronic switch 2 and the load Z can also be connectedin a low-side configuration or in any other configuration as well. Forexample, in a low-side configuration the electronic switch is connectedbetween the load Z and the ground node GND.

According to the example of FIG. 1 , the load Z may be connected to theelectronic switch 2 via an electrically conductive wire (e.g. includedin a cable). Dependent on where the electronic circuit and therespective load Z are located in the electric installation of anautomobile, the wire may have a considerable length of several 10 cm oreven significantly more (e.g. up to 10 m). A modern automobile includesa plurality of electric loads, so that a plurality of wires are requiredto connect the individual loads to their respective electronic switches.In order to save costs and resources, it may be desirable to dimensionthe individual wires such that, in the long term, they withstand anominal current of the connected load. If, however, the current risesabove the nominal current, the wire may be damaged or even destroyed dueto overheating. According to one exemplary embodiment, the controlcircuit 1 may therefore have a current monitor function in order tomonitor the load current i_(L) passing through the electronic switch 2(and the load Z). The current monitoring allows to switch off theelectronic switch 2 in order to protect the wire (and the load Z) whenan “overload situation” is detected. An overload situation is asituation that may result in the wire or the load being damaged ordestroyed if the electronic switch 2 is not switched off (within aspecific time) to disconnect the wire (and the load Z) from the powersource that provides the supply voltage V_(B) (e.g. the automobilebattery). This mechanism is explained in further detail below. As theelectronic fuse circuit F is configured to switch on and off the load Zand to protect the wire, it is also referred to as switching andprotection circuit in the following.

In the example of FIG. 1 , the electronic switch 2 is schematicallydrawn as a circuit block that includes a switch. In the following, theterm “electronic switch” includes any type of electronic switch orelectronic circuitry that has a control node 21 and a load current pathbetween the first load node 22 and the second load node 23 and that isconfigured to be switched on and off dependent on a drive signalreceived at the control node 21. “Switched on” means that the electronicswitch 2 operates in an on-state in which the electronic switch 2 iscapable of conducting a current between the first load node 22 and thesecond load node 23. “Switched off” means that the electronic switch 2is operated in an off-state in which the electronic switch 2 is capableof preventing a current flow between the first load node 22 and thesecond load node 23. According to one example, the electronic switch 2includes at least one transistor. The at least one transistor may be,for example, a MOSFET (Metal Oxide Semiconductor Field-EffectTransistor), an IGBT (Insulated Gate Bipolar Transistor), a JFET(Junction Field-Effect Transistor), a BJT (Bipolar Junction Transistor),or a HEMT (High Electron Mobility Transistor).

In the following, examples of the control circuit 1 and its function areexplained with reference to drawings. In particular, the function of thecontrol circuit 1 is explained with reference to functional blocksdepicted in the drawings. It should be noted that these functionalblocks represent the function of the control circuit 1 rather than itsspecific implementation. These functional blocks may be dedicatedcircuit blocks configured to perform the respective function explainedbelow. However, it may also be possible for the functions of theindividual functional blocks to be (at least in part) performed by aprogrammable circuit (e.g. a processor) that is configured to executesoftware/firmware stored in a memory.

FIG. 2 illustrates one exemplary implementation of the control circuit1. In this example, the control circuit 1 includes a monitoring circuit4 that is configured to generate a first protection signal OC based on atime-current characteristic of the load current i_(L). The expression“to generate the first protection signal OC based on the time-currentcharacteristic of the load current” may include that the monitoringcircuit 4 processes a signal representing the instantaneous currentamplitude of the load current i_(L) as well as previous currentamplitudes to generate the first protection signal OC. That is, themonitoring circuit 4 evaluates a load current i_(L) over a certain timeperiod in order to generate the first protection signal OC. In order tobe able to evaluate the load current i_(L), the monitoring circuit 4receives a current sense signal CS and generates the first protectionsignal OC based on the current sense signal CS. The current sense signalCS represents the load current i_(L) and, according to one example, maybe proportional to the load current i_(L). In the example of FIG. 2 ,the current sense signal CS is available at a sense output 24 of theelectronic switch 2. In this case, a current measurement circuitconfigured to measure the load current i_(L) and provide the currentsense signal CS may be (at least partially) integrated in the electronicswitch 2. However, this is only an example. A current measurementcircuit separate from the electronic switch 2 may be used as well.Various current sense circuits (e.g. shunt resistors, Sense-FETcircuits, etc.) are known and are thus not further explained herein indetail.

The control circuit 1 illustrated in FIG. 2 is configured to drive theelectronic switch 2 based on the protection signal OC and an inputsignal S_(IN) received at a first input node (e.g. input pin) P_(IN) ofthe electronic fuse circuit F. The protection signal OC, as well as theinput signal S_(IN), are supplied to a logic circuit 3, which generatesa drive signal S_(ON) based on the protection signal OC and the inputsignal S_(IN). The drive signal S_(ON) is directly or indirectly (e.g.via diver circuit 5) supplied to the control node 21 of the electronicswitch 2 in order to switch the electronic switch 2 on or off. Accordingto one example, the drive signal S_(ON) may be a logic signal that hasan on-level indicating that it is desired to switch the electronicswitch 2 on or an off-level indicating that it is desired to switch theelectronic switch 2 off. The driver circuit 5 (or simply driver) isconfigured to drive the electronic switch 2 based on the respectivesignal level of the drive signal S_(ON). The electronic switch 2, forexample, includes a transistor such as a MOSFET (as schematicallyillustrated in FIG. 2 ). A MOSFET is a voltage-controlled semiconductordevice that switches on or off dependent on a drive voltage appliedbetween a gate node and a source node. In this example, the driver 5 isconfigured to generate the drive voltage (gate voltage V_(G)) based onthe drive signal S_(ON) in order to switch on or off the MOSFET inaccordance with the drive signal. When using MOSFETs, the driver 5 isalso referred to as gate driver.

The circuit of FIG. 3 illustrates one exemplary implementation of (apart of) the logic circuit 3. In the present example, the logic circuit3 includes an inverter 33, an SR latch 31 (flip-flop) and an AND gate32. A first input of the AND gate 32 is configured to receive the inputsignal S_(IN), whereas a reset input R of the SR latch 31 is configuredto receive the inverted input signal provided by inverter 33. The setinput S of the SR latch 31 is configured to receive the protectionsignal OC. The inverting output Q′ of the SR latch 31 is connected witha second input of the AND gate 32. The drive signal S_(ON) is providedat the output of the AND gate 32.

The function of the logic circuit 3 is further illustrated in the timingdiagrams of FIG. 4 . The initial low level of input signal S_(IN) causesa reset of the SR latch 31, which results in a high level at theinverting output Q′ of the SR latch 31. Accordingly, both inputs of theAND gate 32 “see” a high level, and the output of the AND gate 32provides the drive signal S_(ON) with a high-level. When the inputsignal S_(IN) changes to a low level (indicating a switch-off of theelectronic switch 2, see FIG. 4 , time instant t₁ and t₂), the AND gate32 “sees” a low level at its first input and the output of the AND gate32 provides the drive signal S_(ON) with a low-level (which causes aswitch off of the power transistor 2). In other words, the input signalS_(IN) is fed through the logic circuit 3 (i.e. the drive signal S_(ON)equals the input signal S_(IN)) provided that the SR latch 31 is in itsreset state. Once the SR latch 31 is set in response to the protectionsignal OC changing to a high-level, the inverting output Q′ of the SRlatch 31 is set to a low level (see FIG. 4 , time instant t₃).Accordingly, the AND gate 32 sees a low level at its second input andthe drive signal S_(ON) is set to a low level. In other words, the inputsignal S_(IN) is blanked by the AND gate 32. The drive signal S_(ON)remains at a low-level until the input signal S_(IN) is set to a lowlevel (indicating a switch-off of the electronic switch 2 and a reset ofSR latch 31, see FIG. 4 , time instant t₄) and again to a high level(indicating a switch-on of the electronic switch 2, see FIG. 4 , timeinstant t₅). It is again noted that the function of the exemplaryimplementation of FIG. 3 may be implemented in various other ways aswell. Further, it is noted that in other embodiments the reset of the SRlatch 31 may be triggered in a different way. For example, themicrocontroller 8 (see FIG. 1 ) may provide a dedicated reset signal.

As mentioned above, the wire connecting load Z and electronic fusecircuit F can be designed to withstand a nominal current of the load Z.The lifetime of a wire (or a cable) depends on the wire temperature.FIGS. 5A and 5B are diagrams showing a family of characteristic curves(each representing a specific time-current characteristics), whereineach characteristic curve is associated with a specific combination ofmaximum temperature difference dT (maximum temperature above ambienttemperature) and cable cross section (e.g. cross-sectional area in mm²).Each characteristic curve can be regarded as an “isotherm” (line ofequal temperature dT) and represents the relation between the currentand the maximum allowable time period that the wire can carry thecurrent without exceeding the specified temperature difference dT.

FIG. 5A shows characteristic curves for various temperature differencesdT and a specific cross sectional area of 0.35 mm², while FIG. 5B showscharacteristic curves for a specific temperature difference dT of 25 K(Kelvin) and various cross sectional areas. As can be seen from FIGS. 5Aand 5B, a wire with a cross-sectional area of 0.35 mm² can carry acurrent of approximately 9 A (amperes) for a practically infinite amountof time without exceeding a temperature difference dT of 25 K aboveambient temperature. As can be seen from FIG. 5B, a wire with across-sectional area of 0.75 mm² can carry a current of 10 A (amperes)for approximately 100 seconds or 35 A for approximately 1 second beforeexceeding a temperature difference dT of 25 K above ambient temperature.Generally the higher the current, the shorter the allowable time periodfor a given cross-sectional area and a given temperature difference. Itis noted that the characteristic curves shown in the diagrams of FIGS.5A and 5B have a linearly falling branch in a double logarithmicrepresentation.

As can be seen from FIGS. 5A and 5B, a temperature difference dT_(x)(e.g. temperature values dT₁, dT₂, dT₃, dT₄, dT₅, dT₆) is associatedwith a given integration time t_(x)(e.g. times t₁, t₂, t₃, t₄, t₅, t₆)for a given current (see FIG. 5A, current i_(x)) and a specificcross-sectional area (e.g. 0.35 mm² in the example of FIG. 5A). Hence, atemperature value dT (representing the temperature above ambienttemperature) can be determined for a specific wire cross section byintegrating the power resulting from a load current i_(L)=i_(x) passingthrough the wire over time. The first protection signal OC may indicatea switch-off of the electronic switch 2 when the temperature value dTreaches a defined first reference temperature difference dT_(R). Thementioned integration can be efficiently implemented using a digitalfilter, which may be included in the monitoring circuit 4 (see FIG. 2 ).One exemplary implementation of a monitoring circuit is illustrated inFIG. 6 . In the embodiments described herein, the digital filter is alow-pass filter. In one embodiment, the low-pass filter is a first orderfilter, which is sufficient when using a simple thermal model of thecable (based on Fourier's law).

Basically, the monitoring circuit of FIG. 6 is configured to determinethe first protection signal OC based on the current sense signal CS. Asmentioned, the integration can be carried out in a digital filter 42,which has an integrating characteristic (implemented by a low-passfilter). According to the depicted example, the current sense signal CS,which may be a voltage that is proportional to the load current i_(L),is supplied to the input of filter 45, which may be an (optional) analoglow-pass filter, to remove transients or the like that have a comparablyhigh frequency. The output of filter 45 may be connected to the input ofanalog-to-digital converter (ADC) 41, which is configured to digitizethe filtered current sense signal CS. The ADC 41 may have a logarithmiccharacteristic in order to account for the logarithmic characteristiccurves shown in FIGS. 5A and 5B. The (e.g. logarithmized) digitalcurrent sense signal CS_(DIG) is then “transformed” to a temperaturevalue dT by digital filter 42. The resulting temperature value dT(representing a temperature difference above ambient temperature) isthen supplied to digital comparator 43, which can be configured to setthe first protection signal OC to a high-level when the temperaturevalue dT provided at the output of digital filter 42 exceeds the firstreference temperature difference dT_(R) (e.g. 25 K) specified for aspecific wire cross-section.

The squaring unit 46 depicted in FIG. 6 can be omitted, dependent on thecharacteristic of the ADC 41. However, if the ADC 41 has a “normal”(i.e. linear) characteristic, the squaring is needed to obtain a valueindicative of the power. In other embodiments, the squaring unit 46, ifomitted, should be replaced by other suitable non-linear functions. Inessence, the input signal supplied to filter 42 is representative of thepower resulting from the load current i_(L).

As mentioned, the digital filter 42 is configured to convert the (e.g.squared) load current and an associated integration time, during whichthe current passes through the wire, into a temperature value dT. In thepresent example, the filter characteristic 42 depends on a parametercharacterizing the wire, e.g. the cross-sectional area of the wire thatcarries the current, and may be represented by a family ofcharacteristic curves such as those shown in the diagram of FIG. 5A (foran exemplary cross-sectional area of 0.35 mm²).

FIG. 7 illustrates one example of an electronic fuse circuit, which isfurther referred to as smart fuse circuit 10. The circuit of FIG. 7 issubstantially the same as the circuit of FIG. 2 and reference is made tothe respective description. However, the logic circuit 3 is moresophisticated than in the example of FIG. 2 and the monitoring circuit 4is implemented in accordance with FIG. 6 , wherein the analog low-passfilter 45 has been omitted (the low-pass filter 45 is optional).However, different from the example in FIG. 6 , the monitoring circuit 4is configurable in the present example such that its characteristic canbe selected based on at least one wire parameter, which allows, forexample, to select a characteristic for a specific wire cross sectionand/or a desired reference temperature difference dT_(R) (temperaturethreshold). In the examples described herein, the at least one wireparameter represents the cable cross-sectional area and/or the maximumtemperature value above ambient temperate. As can be seen in thediagrams of FIGS. 5A and 5B, these two wire parameters define a specificcharacteristic curve that represents the desired behavior of theelectronic fuse circuit for a specific wire/cable. It is understood thatother parameters such as wire diameter or absolute temperature (e.g. incase ambient temperature is measured) can be used as wire parameters.Furthermore, a wire parameter is not necessarily representative of anyphysical quantity (such as cross-sectional area or temperature) but canbe a mere numerical parameter that allows determining (e.g. selecting)the desired characteristic used by the monitoring circuit. In oneexample, the wire parameter is merely a number indicating thecharacteristic curve to be applied. As shown in FIG. 7 , the electronicfuse circuit may be an integrated circuit arranged in one chip package,wherein the electronic switch 2 and the remaining circuit components(driver 5, logic circuit 3 and monitoring circuit 4) may be integratedin the same semiconductor die or in two separate semiconductor diesdisposed in the chip package. However, in other embodiments the smartfuse circuit 10 may be distributed in two or more separate chippackages. In the example of FIG. 7 all the depicted circuit componentsare integrated in one semiconductor chip.

The load current path of the electronic switch 2 may be connectedbetween a supply pin SUP and an output pin OUT of the smart fuse circuit10. Generally, the logic circuit 3 may be configured to receive at leastone wire parameter, which in the present example includes informationabout a wire cross-sectional area A and a reference temperaturedifference dT_(R), from a microcontroller or other control circuitry. Asillustrated in FIG. 6 , the logic circuit 3 may be configured to receivesignals from a controller via input pin IN (input signal S_(IN), seealso FIG. 2 ) and input pins SEL_(WIRE) and SEL_(dT) (selection signalsS_(s1) and S_(S2) representing a wire cross-sectional area and atemperature difference) and to provide a drive signal S_(ON) for theelectronic switch 2. The driver 5 may be configured to convert thesignal S_(ON), which is a binary logic signal, into a drive voltage ordrive current suitable to switch the electronic switch 2 on and off. Asin the example of FIG. 2 , the monitoring circuit 4 receives an (analog)current sense signal CS and generates, based on this current sensesignal CS, the first protection signal OC, which may be processed by thelogic circuit 3, for example, as shown in the example of FIG. 3 .

As mentioned, the filter 42 can be implemented as a first-order low-passfilter. That is, the (continuous-time) filter transfer function H(s) canbe written as follows:

$\begin{matrix}{{H(s)} = \frac{b}{1 + {s\tau}}} & (1)\end{matrix}$wherein τ represents the filter time constant and b represents thefilter gain. The comparator 43 triggers a switch-off of the electronicswitch 2 (by generating an over-current signal OC) when the followingcondition is fulfilled:

$\begin{matrix}{{\mathcal{L}^{- 1}\left\{ {{P(s)}\frac{b}{1 + {s\tau}}} \right\}} \geq {\Delta\;{T.}}} & (2)\end{matrix}$That is, a switch-off is triggered when the estimated cable temperature,which is represented by the filter output of the filter 42, reaches orexceeds a temperature threshold ΔT (in equation 2

⁻¹{·} denotes the inverse Laplace transform). The above condition (2)can be reformulated as

$\begin{matrix}{{{\mathcal{L}^{- 1}\left\{ {{P(s)}\frac{1}{1 + {s\tau}}} \right\}} \geq {dT_{R}}},} & (3)\end{matrix}$wherein dT_(R)=ΔT/b, and P(s) denotes the Laplace transform of thefilter input signal. It is evident from conditions (2) and (3) thatfilter gain b and threshold value ΔT are not independent parameters. Aspecific reference temperature dT_(R) can be achieved by differentcombinations of filter gain b and threshold value ΔT Varying the filtergain b has a similar effect as varying the temperature threshold ΔT. Itis understood that, although dT_(R) represents a temperature, it is notmeasured in Kelvin (as can be seen in FIG. 10 , dT_(R) has the physicaldimension of amperes squared).

FIGS. 8 and 9 illustrate the effect of varying the filter time constantτ and the filter gain b on the characteristic curve (cf. FIGS. 5A and5B). As shown in FIG. 8 , varying the filter time constant τ results ina vertical shift of the characteristic curve due to the scaling of thetime axis. In contrast, as shown in FIG. 9 , varying the filter gain bresults in a horizontal shift of the characteristic curve due to thescaling of the current axis. It is noted that, in the example of FIG. 9, a filter gain b=1 results in a reference temperature dT_(R) of ΔT/b=20degrees Celsius. Similarly, a filter gain b=0.5 results in a referencetemperature dT_(R) of ΔT/b=40 degrees Celsius, a filter gain b=0.2results in a reference temperature dT_(R) of ΔT/b=100 degrees Celsius,and a filter gain b=0.1 results in a reference temperature dT_(R) ofΔT/b=200 degrees Celsius.

It is again emphasized that instead of changing the filter gain b, thereference temperature dT_(R) can be changed to achieve the same effect.In the further description, it is assumed (without loss of generality)that the filter gain b is constant and set to b=1, and the referencetemperature dT_(R) is adjustable to match the specification for aspecific cable.

As mentioned, the filter output signal provided by the filter 42 andsupplied to the comparator input of comparator 43 can be interpreted asa temperature. As can be seen from FIGS. 8 and 9 , the options forselecting a specific time-current characteristic are very limited.Basically, the filter time constant τ and the comparator thresholddT_(R) determine the characteristic curve, which can be shiftedvertically and horizontally by varying the parameters τ and dT_(R).However, by changing these two parameters it is not possible to changethe shape of the time-current characteristic as such.

FIG. 10 illustrates a modified monitoring circuit 4 which allows toadapt the shape of the time-current characteristic in a flexible way andto customize it for a specific application. Like the example of FIG. 6 ,the embodiment of FIG. 8 includes an analog-to-digital converter 41 anda squaring unit 46. The analog-to-digital converter 41 receives thecurrent sense signal CS(t) and provides the corresponding digitizedsignal CS[k] (k being the time index); the squaring unit 46 provides asignal representing the squared signal CS[k]². The squared signal CS[k]²is distributed to a plurality of signal paths, wherein each signal pathincludes a filter 42.n and a comparator 43.n (n=1, 2, . . . N).

The output signals of the filters 42.1, 42.2, . . . , 42.N are denotedas y₁[k], y₂[k], . . . , y_(N)[k]; and the output signals of thecomparators 43.1, 43.2, . . . , 43.N are denoted as OC₁, OC₂, . . . ,OC_(N). These over-current signals OC₁, OC₂, . . . , OC_(N) are suppliedto a logic circuit 47, which may be an OR-gate with multiple inputs. Thecomparator thresholds are denoted as dT_(R,1), dT_(R,2), . . . ,dT_(R,N). The output of the logic circuit 47 is denoted as OC[k] andsignals a fault (which may trigger a switch-off of the electronic switch2) when one of the over-current signals OC₁, OC₂, . . . , OC_(N)indicates a violation of the respective comparator threshold. In theexample of FIG. 10 , all filters 42.1, 42.2, . . . , 42.N receive thesquared current sense signal CS[k]², wherein only the output signaly₁[k] is indicative of the physical quantity “cable temperature”.

The example of FIG. 11 is almost identical to the previous example ofFIG. 10 except that only the first filter 42.1 receives the squaredcurrent sense signal CS[k]² whereas the other filters 42.2, . . . , 42.Nreceive the (non-squared) current sense signal CS[k]. As mentioned, onlythe output signal y₁[k] of the first filter 42.1 is indicative of thecable temperature whereas the remaining filters 2.2, . . . , 42.N andcorresponding comparator thresholds dT_(R,2), . . . , dT_(R,N) aremerely needed to adapt the time-current characteristic in a desiredmanner so that it meets the requirements of a specific application. Oneexample is illustrates in FIG. 12 . According to this, the resultingtime-current characteristic is a concatenation of segments of thecharacteristic curves resulting from the N signal paths (for N=3 in thedepicted example). Due to the OR-conjunction provided by the logiccircuit 47, the resulting time-current characteristic is determined bythe minimum of the characteristic curves of the N signal paths. That is,when one of the N signal paths signals an over-current switch-off(over-current signals OC₁, OC₂, . . . , OC_(N)) then the signal OC willtrigger a switch-off.

FIG. 13 illustrates a further embodiment which can be seen as anenhancement of the example of FIG. 10 . The circuit of FIG. 13 isbasically the same as the circuit in FIG. 10 but with additionalcircuitry for determining the remaining minimum headroom before anovercurrent signal OC[k] (which usually triggers a switch-off) isgenerated. Accordingly, the circuit of FIG. 13 includes calculationcircuits (e.g. subtractors) 43.1, 43.2, . . . , 43.N configured togenerate output signals h₁[k], h₂[k], . . . , h_(N)[k] representing thedifferences y₁[k]-dT_(R,1), y₂[k]-dT_(R,2), . . . , y_(N)[k]-dT_(R,N)“seen” by the comparators 43.1, 43.2, . . . , 43.N. The selectioncircuit 49 is configured to provide, as output signal h_(min)[k], theinstantaneous minimum of the signals h₁[k], h₂[k], . . . , h_(N)[k],i.e. h_(min)[k]=min{h₁[k], h₂[k], . . . , h_(N)[k]}. The signalh_(min)[k] is representative of the available headroom before a failureis signaled (by setting the level of signal OC[k] to an appropriatevalue). As can be seen from FIG. 13 , the circuit is capable ofproviding three types of information to be processed. One suchinformation is the signal OC[k], which indicates an overcurrent andusually triggers a switch-off of the electronic switch 2 (cf. FIG. 2 ).Triggering a switch-off in response to the signal OC[k] is analogous toa conventional fuse being triggered in response to an overcurrent.Further information can be the headroom signal h_(min)[k], which isindicative of how close the monitoring circuit is to generating a signalOC[k] indicating an overcurrent. Third, the cable temperature increase(temperature difference to ambient temperature) or the equivalentthermal status calculated by the filter 43.1 can also be provided.

The monitoring circuits of FIGS. 10, 11, and 13 may be integrated in onesemiconductor chip package as discussed with reference to FIG. 7 . Anexemplary application is illustrated in FIG. 14 , in which theintegrated circuit, including electronic switch 2, monitoring circuit 4,and logic circuit 3 (see also FIGS. 2 and 7 ), is referred to asintegrated smart fuse circuit 10. The input signal S_(IN) is provided,in the present example, by a microcontroller 8. Further, the smart fusecircuit 10 is configured to provide the current sense signal CS(t) orany other signal representative of the load current passing through theelectronic switch 2. This current sense signal CS(t) is received anddigitized by the microcontroller 8.

With a circuit structure as shown in FIG. 14 , it is possible toimplement an additional filter (referred to as filter 42.N+1) in themicrocontroller 8. Accordingly, the microcontroller 8 digitizes thecurrent sense signal CS(t) and feeds the digitized signal CS[k] into adigital filter (e.g. a low-pass filter as discussed above with referenceto FIG. 10 ). The filtered signal y_(N+1)[k] is compared with areference value dT_(R,N+1) and the input signal S_(IN)′ is blanked ifthe filtered signal y_(N+1)[k] reaches or exceeds the reference valuedT_(R,N+1). The modified (i.e. blanked, as the case may be) input signalis denoted as S_(IN)′ in FIG. 14 . That is, due to the blanking of theinput signal S_(IN), the electronic switch 2 is switched off analogouslyas it would be in response to the signal OC[k] signaling an overcurrent.Accordingly, in this example, the logic circuit 3 (see FIGS. 2 and 7 )is partly translocated into the microcontroller 8.

It is understood that the additional feedback loop implemented in themicrocontroller 8 in the example of FIG. 14 may also be implemented byother external circuitry (i.e. outside the smart fuse circuit 10) otherthan the microcontroller.

The following numbered clauses demonstrate one or more aspects of thedisclosure.

Clause 1—A circuit comprising: a monitor circuit (4) configured toreceive a current sense signal (CS[k]) and to provide a protectionsignal (OC), wherein the monitor circuit (4) comprises: a nonlinearfunctional unit configured to receive the current sense signal (CS[k])and to generate a power signal (CS[k]²) representing the power of thecurrent sense signal; a first filter configured to receive the powersignal (CS[k]²) and to generate a first filtered signal (y₁[k]), and asecond filter configured to receive an input signal that depends on thecurrent sense signal (CS[k]) and to generate a second filtered signal(y₂[k]); and a comparator circuit configured to receive the firstfiltered signal (y₁[k]) and the second filtered signal (y₂[k]) and tocompare the first filtered signal (y₁[k]) with a first threshold value(ΔT₁) and the second filtered signal (y₂[k]) with a second thresholdvalue (ΔT₂); the protection signal being indicative of whether the firstfiltered signal (y₁[k]) exceeds the first threshold value (ΔT₁) or thesecond filtered signal (y₂[k]) exceeds the second threshold value (ΔT₂).

Clause 2—The circuit of clause 1, further comprising: an electronicswitch (2) coupled between a supply pin (SUP) and an output pin (OUT);and a current sensing circuit coupled to the electronic switch (2) andconfigured to generate the current sense signal (CS[k]) indicative of aload current (i_(L)) passing through the electronic switch (2).

Clause 3—The circuit of clause 1 or 2, wherein the first filter is alow-pass filter.

Clause 4—The circuit of any of clauses 1 to 3, wherein the first filteris a first order low-pass filter.

Clause 5—The circuit of any of clauses 1 to 4, wherein the input signalof the second filter is the power signal (CS[k]²).

Clause 6—The circuit of any of clauses 1 to 4, wherein the input signalof the second filter is the current sense signal (CS[k]).

Clause 7—The circuit of any of clauses 1 to 6, wherein the currentsensing circuit includes an analog-to-digital converter configured toprovide the current sense signal (CS[k]) in digital form; and whereinthe nonlinear functional unit is implemented in the analog-to-digitalconverter by using a non-linear analog-to-digital conversioncharacteristics.

Clause 8—The circuit of any of clauses 1 to 7, wherein the nonlinearfunctional unit is configured to perform a digital squaring of thecurrent sense signal (CS[k]).

Clause 9.—The circuit of any of clauses 1 to 7, wherein the nonlinearfunctional unit is configured to generate the power signal (CS[k]²) as asignal proportional to the square of the load current (i_(L)).

Clause 10—The circuit of any of clauses 2 to 9, if referring to claim 2,wherein the first filtered signal (y₁[k]) represents a temperaturedifference of a cable connected to the electronic switch relative toambient temperature.

Clause 11—The circuit of any of clauses 1 to 10, wherein the monitorcircuit (4) is further configured to generate a headroom signal (h[k])based on the differences between the filtered signals (y₁[k], y₂[Mk], .. . , y_(N)[k]) supplied to the comparator circuit and the respectivethreshold values (dT_(R1), dT_(R2), . . . , dT_(RN)).

Clause 12—The circuit of any of clauses 1 to 11, if referring to claim2, further comprising: a logic circuit (3) configured to trigger aswitch-off of the electronic switch (2) or signal an error in responseto the protection signal (OC).

Clause 13—The circuit of clause 12, wherein the logic circuit (3) isconfigured to receive a switch-on command and to trigger a switch-on ofthe electronic switch (2) in response to the switch-on command.

Clause 14—The circuit of any of clauses 1 to 13, if referring to claim2, wherein the current sensing circuit, the nonlinear functional unit,and the first filter are integrated in a single semiconductor chip, thesemiconductor chip has an output contact configured to provide a signalrepresenting the current sense signal (CS(t)), and wherein the secondfilter is implemented using external circuitry connected to thesemiconductor chip.

Clause 15—The circuit of any of clauses 1 to 14, further comprising: atleast a third filter configured to receive the input signal that dependson the current sense signal (CS[k]) and to generate a third filteredsignal (y₃[k]), wherein the comparator circuit is further configured toreceive the third filtered signal (y₃[k]) and to compare it with a thirdthreshold value (ΔT₃); the protection signal being indicative of whetherone of the filtered signals (y₁[k], y₂[k], y_(N)[k]) exceeds therespective threshold value (ΔT₁, ΔT₂, . . . ΔT_(N)).

Clause 16—A method comprising: providing a signal representing a loadcurrent (i_(L)) passing through a power transistor (2); and generating aprotection signal (OC) based on the current sense signal (CS); whereingenerating the protection signal (OC) includes: generating a powersignal (CS[k]²) representing the power of the current sense signal byapplying a nonlinear function to the current sense signal (CS);filtering the power signal (CS[k]²) to generate a first filtered signal(y₁[k]) and generating a second filtered signal (y₂[k]) based on thecurrent sense signal (CS); and comparing the first filtered signal(y₁[k]) with a first threshold value (dT_(R1)) and the second filteredsignal (y₂[k]) with a second threshold value (dT_(R2)); the protectionsignal (OC[k]) being indicative of whether the first filtered signal(y₁[k]) exceeds the first threshold value (ΔT₁) or the second filteredsignal (y₂[k]) exceeds the second threshold value (ΔT₂).

Clause 17—The method of clause 16, further comprising: disconnecting anoutput pin (OUT) from a supply pin (SUP) using the power transistorbased on the protection signal (OC[k]).

Although the invention has been illustrated and described with respectto one or more implementations, alterations and/or modifications may bemade to the illustrated examples without departing from the spirit andscope of the appended claims. In particular regard to the variousfunctions performed by the above described components or structures(units, assemblies, devices, circuits, systems, etc.), the terms(including a reference to a “means”) used to describe such componentsare intended to correspond—unless otherwise indicated—to any componentor structure, which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure, which performs thefunction in the herein illustrated exemplary implementations of theinvention.

The invention claimed is:
 1. A circuit comprising: a monitor circuitconfigured to receive a current sense signal and to provide a protectionsignal, wherein the monitor circuit comprises: a nonlinear functionalunit configured to receive the current sense signal and to generate apower signal representing the power of the current sense signal; a firstfilter configured to receive the power signal and to generate a firstfiltered signal; a second filter configured to receive an input signalthat depends on the current sense signal and to generate a secondfiltered signal; a third filter configured to receive the input signalthat depends on the current sense signal and to generate a thirdfiltered signal; and a comparator circuit configured to receive thefirst filtered signal and the second filtered signal and the thirdfiltered signal and to compare the first filtered signal with a firstthreshold value and to compare the second filtered signal with a secondthreshold value and to compare the third filtered signal with a thirdthreshold value; the protection signal being indicative of whether thefirst filtered signal exceeds the first threshold value or the secondfiltered signal exceeds the second threshold value or the third filteredsignal exceeds the third threshold value.
 2. The circuit of claim 1,further comprising: an electronic switch coupled between a supply pinand an output pin; and a current sensing circuit coupled to theelectronic switch and configured to generate the current sense signalindicative of a load current passing through the electronic switch. 3.The circuit of claim 2, wherein the first filtered signal represents atemperature difference of a cable connected to the electronic switchrelative to ambient temperature.
 4. The circuit of claim 2, furthercomprising: a logic circuit configured to trigger a switch-off of theelectronic switch or signal an error in response to the protectionsignal.
 5. The circuit of claim 4, wherein the logic circuit isconfigured to receive a switch-on command and to trigger a switch-on ofthe electronic switch in response to the switch-on command.
 6. Thecircuit of claim 2, wherein the current sensing circuit, the nonlinearfunctional unit, and the first filter are integrated in a singlesemiconductor chip, the semiconductor chip has an output contactconfigured to provide a signal representing the current sense signal,and wherein the second filter is implemented using external circuitryconnected to the semiconductor chip.
 7. The circuit of claim 1, whereinthe first filter is a low-pass filter.
 8. The circuit of claim 1,wherein the first filter is a first order low-pass filter.
 9. Thecircuit of claim 1, wherein the input signal of the second filter is thepower signal.
 10. The circuit of claim 1, wherein the input signal ofthe second filter is the current sense signal.
 11. The circuit of claim1, wherein the current sensing circuit includes an analog-to-digitalconverter configured to provide the current sense signal in digitalform; and wherein the nonlinear functional unit is implemented in theanalog-to-digital converter by using a non-linear analog-to-digitalconversion characteristics.
 12. The circuit of claim 1, wherein thenonlinear functional unit is configured to perform a digital squaring ofthe current sense signal.
 13. The circuit of claim 1, wherein thenonlinear functional unit is configured to generate the power signal asa signal proportional to the square of the load current.
 14. The circuitof claim 1, wherein the monitor circuit is further configured togenerate a headroom signal based on the differences between the filteredsignals supplied to the comparator circuit and the respective thresholdvalues.
 15. A method comprising: providing a signal representing a loadcurrent passing through a power transistor; and generating a protectionsignal based on the current sense signal; wherein generating theprotection signal includes: generating a power signal representing thepower of the current sense signal by applying a nonlinear function tothe current sense signal; filtering the power signal to generate a firstfiltered signal and generating a second filtered signal based on thecurrent sense signal; generating a third filtered signal using a thirdfilter to filter the current sensed signal; and comparing the firstfiltered signal with a first threshold value, comparing the secondfiltered signal with a second threshold value, and comparing the thirdfiltered signal with a third threshold value; the protection signalbeing indicative of whether the first filtered signal exceeds the firstthreshold value or the second filtered signal exceeds the secondthreshold value or the third filtered signal exceeds the third thresholdvalue.
 16. The method of claim 15, further comprising: disconnecting anoutput pin from a supply pin using the power transistor based on theprotection signal.